In the receiving apparatus of a semiconductor integrated circuit for radio use as processes an RF (Radio Frequency) signal which is used in a portable terminal or the like, a superheterodyne scheme and a direct conversion scheme have been generally known as methods for a frequency conversion from an RF bandwidth into a baseband frequency bandwidth (low-frequency bandwidth). The superheterodyne scheme is a scheme wherein a reception frequency is converted by performing a plurality of times of downconversions as from the RF bandwidth into an intermediate frequency (IF) bandwidth and from the IF bandwidth into the baseband frequency bandwidth. On the other hand, the direct conversion scheme is a scheme wherein the RF bandwidth is frequency-converted directly into the baseband frequency bandwidth by one time of downconversion.
When compared with the heterodyne scheme which uses the IF bandwidth, the direct conversion scheme can decrease high-frequency circuits, IF filters, etc. Therefore, this scheme has the advantages of reduction in size, etc. and is often applied presently to the high-frequency ICs (Integrated Circuits), etc. of the portable terminal. The direct conversion scheme, however, has the problem that a signal deterioration is incurred by the fluctuation (DC offset) of a DC (Direct Current) attributed to flicker noise or self-mixing. Here, the flicker noise is noise which exists in the vicinity of the DC and which increases in inverse proportion to the frequency. The deterioration of a desired signal due to the DC offset is not limited to the direct conversion scheme, and it becomes especially problematic. Moreover, the settling time period of a transient signal as arises in case that a gain has been switched in a baseband unit for processing a baseband signal, sometimes becomes problematic. It is known that a high-pass filter (HPF) is generally employed in order to eliminate the DC offset mentioned above. The HPF removes the low-frequency bandwidth, and its cutoff frequency needs to be made sufficiently low for the purpose of preventing the deterioration of the desired signal which exists in a near-DC bandwidth. A capacitance and a resistance of large values as constitute the HPF are required for making the cutoff frequency sufficiently low. In general, the capacitance and the resistance which have the large values, also enlarge physically and therefore hamper reduction in the size of the HPF. Furthermore, in the case that the capacitance and the resistance having the large values are employed as those constituting the HPF, the time constant of a circuit enlarges, and hence, a long time is expended till the settling of the gain.
Patent Document 1 being JP-A-2005-286810 discloses a method wherein, in a radio terminal of direct conversion scheme, the cutoff frequency of an HPF is switched in order to perform a high-speed operation while eliminating a DC offset. More specifically, the cutoff frequency is set high for shortening a gain settling time period, and it is set low in a period for which the deterioration of a desired signal is prevented, whereby the high-speed operation can be performed with the DC offset eliminated.